1. Field Of The Invention
In one aspect this invention relates to devices for measuring the presence of selected gas or gases in a gas mixture. In another aspect this invention relates to a device suitable for measuring the time-average exposure of an individual to a selected gas in air.
2. Prior Art
One example of a detection device which measures the presence of a contaminant vapor is U.S. Pat. No. 3,714,562 issued to McNerny which discloses the use of a metallic film to absorb a selected vapor and the presence of the vapor is measured by a change in resistance. Also disclosing resistance-type sensing elements are U.S. Pat. Nos. 3,703,696; 2,976,188 and 2,713,625. These prior art detectors pass a moving vapor over the resistance element making them dependent on a constant velocity for accuracy. Such devices do not function properly when attached to a person such that the gas flow over the sensor is dependent upon the person's movement. Because people have a variation in movement, and the air movement in a room may also vary, a simple badge sensor which permits the access of air to a sensing element when attached to a person, will vary greatly in response as a function of velocity making accurate monitoring of an individual's exposure impossible.
A further detection system is disclosed by Palmes and Gunnison, "Personal Monitoring Device for Gaseous contaminants," American Industrial Hygiene Association Journal, V. 34, No. 2, February, 1973, pp. 78-81. This device measures concentrations by measuring the quantity of a selected gas which diffuses through a single orifice of known size into a chamber maintained at zero concentration of the selected gas by means of a collecting medium.